1. Field of the Invention
The present invention relates to a sheet convey apparatus for conveying a sheet.
2. Related Background Art
In recording apparatuses such as printer, copying machine, facsimile machine and the like, an image is recorded on a recording sheet (recording medium) such as a paper sheet, a plastic film and the like by selectively driving energy generating elements of a recording head in response to recording information. The recording apparatus can be grouped into an ink jet type, a wire dot type, a thermal type, a laser beam type or the like in accordance with recording systems. Further, the recording sheet used with the recording apparatus may be not only a normal sheet, but also a thick sheet such as a post card and an envelope or a special sheet such as a plastic film.
As shown in FIG. 9, in a conventional recording apparatus, recording sheets 117 are separated one by one by rotating supply rollers 108 of an automatic sheet supply portion 105, and the separated sheet is fed to a sheet convey apparatus having a convey roller 103 and pinch rollers 107 and then is fed to a recording portion from the convey roller 103. Thereafter, the recording sheet is sent to sheet discharge rollers disposed rearwardly of the convey roller 103 and then is discharged onto a discharge tray.
The conveyance of the recording sheet 117 is effected by urging the recording sheet 117 against the convey roller 103 by a plurality of pinch rollers 117 and by rotating the convey roller 103. A surface of the convey roller 103 is formed from material having high coefficient of friction such as rubber or urethane and a surface of each pinch roller 117 is formed from material having low coefficient of friction such as SUS or Teflon. Each pinch roller 117 is rotatably mounted on both ends 114a, 114b of a corresponding pinch roller holder member 114. The other end of the pinch roller holder member 114 is rotatably mounted on a chassis 116 and is biased by spring members 109 to urge the pinch roller 107 against the convey roller 103. In order to urge the pinch roller against the convey roller, there have been proposed a method for urging the other end of the bearing of the pinch roller 107 as better dynamic point with respect to the rotation center of the pinch roller holder 114 by a leaf spring, a coil spring and the like, a method for directly urging the bearings at left and right ends of the pinch roller holder as better dynamic points, a method for urging the center of the pinch roller holder and the like.
However, in the conventional sheet convey apparatus, since the bearings of the pinch roller holder are disposed at two points (left and right ends of the pinch roller), the urging forces at the left and right ends of the pinch roller (against the convey roller) are differentiated from each other due to the dispersion in the dimensions of the bearings and the dispersion in the spring forces, with the result that, if the recording sheet is conveyed only by the single pinch roller, the sheet will be skew-fed. Further, since the bearings of the pinch roller holder 114 are disposed at two left and right points, it is feared that the urging force is decreased due to the elastic deformation of the pinch roller holder. In order to prevent the elastic deformation, an additional bearing is must be provided at a center of the pinch roller holder 114. However, in this case (when three bearings are arranged), since the sliding resistance of each pinch roller is increased, the convey accuracy of the recording sheet is badly influenced.